1. Field of Invention
This invention relates to telephone subscriber loop interface circuits (SLICs) and more particularly to a monolithic integrated detection circuit for detecting the hook status of a telephone handset loop coupled to the SLIC.
2. Description of the Prior Art
Hybrid or subscriber loop interface circuits (SLICs) are generally known in the art which provide balanced two-wire to unbalanced four-wire conversion between subscriber telephone handsets and the central telephone office equipment. For example, U.S. Pat. No. 4,004,109 and the above referenced Kelley et al patent application describe such SLICs which are comprised of a plurality of complementary current mirror circuits.
Because the SLIC equipment is generally in use a small percentage of the time, it is desirous for the SLIC to include circuitry to detect the inactive ON-hook condition of the telephone handset to power down the SLIC internal circuits to minimize office power supply requirements.
Therefore, a need exists for circuit means to be included with contemporary SLICs for detecting the hook status of the subscriber loop which:
a. accurately measures the subscriber loop resistance
b. prevents false off-hook conditions which may be caused by longitudinal signals appearing at the inputs of the SLIC
c. requires no, or minimal, direct current power when the subscriber loop is open circuited and
d. is independent to power supply voltage variations.